1. Field of the Invention
The present invention relates generally to a sine wave oscillator and, more particularly, to a sine wave oscillator having a self-startup circuit that is adapted to self-oscillate without requiring any input signals other than a supply voltage, and to output sine waves having a constant frequency.
2. Description of the Related Art
A typical oscillator is an oscillator for generating pulses, converting the waveforms of the generated pulses, and outputting sine waves. Such an oscillator is widely used because it is easy to implement the oscillator from the point of view of circuit construction and it is also easy to perform frequency control. However, this oscillator has shortcomings: the construction of circuits is complicated because a waveform converter for outputting sine waves requires a separate resistor, capacitor and amplifier, and in that the extent of distortion of sine waves is relatively high because square waves are converted into sine waves.
As a result, an oscillator, including an operational amplifier, resistor and capacitor, capable of generating sine waves having low waveform distortion is widely used as a sine wave oscillator (see the conventional circuit diagram of FIG. 7).
However, such an R-C oscillator is not able to be oscillated initially merely by the application of supply voltage.
Hence there are many methods for initializing oscillators. One widely used method is the technology of connecting a feedback resistor R4, a parallel resistor R5, and diodes D1 and D2, between the output and negative input terminal of an amplifier, as shown in FIG. 8, thereby facilitating startup.
Referring to the construction and operation of the conventional circuit of FIG. 7, resistors R3 and R4, connected in series to each other, are connected to a negative feedback circuit. Negative feedback gain β is (1+R4/R3), and positive feedback gain K is jw(C1R2/[(1−w2C1C2R1R2)+jw(C1R1+C2R2+C1R2)]. The condition for maintaining oscillation is that a value, obtained by multiplying the positive and negative feedback gains by each other, must be 1, which is expressed by the equation Kβ=1. It can be seen that the equality (1+R4/R3)=(1+C2/C1+R1/R2) must be satisfied.
However, since oscillation can be generated in its early stage only when there is sufficient negative feedback gain, the inequality (1+R4/R3)>>(1+C2/C1+R1/R2) must be satisfied. The conventional circuit of FIG. 7 has no circuit for implementing this, thus initial startup oscillation is difficult to occur.
In FIG. 8, the resistor R5 and the diodes D1 and D2 are added as a circuit construction for facilitating the startup of oscillation in FIG. 7. In this circuit, the forward resistances of the diodes are very low and the diodes D1 and D2 have high resistances because the diodes D1 and D2 are all “OFF” before the start of oscillation, resulting in a large negative feedback gain, which meets the initial startup condition, can be obtained if the values of the resistors R4 and R5 are appropriately selected. Accordingly, using the additional circuits, the startup of oscillation can be easily achieved, and oscillation can be maintained.
However, the conventional circuit has a problem in that the generated frequency value varies after initial oscillation because the forward resistance value of the diode varies with the magnitude of voltage across both ends of the diode. Accordingly, the conventional circuit is not appropriate for an accurate frequency oscillator.
Although further methods and circuits for generating and outputting sine waves having desired frequencies using various technologies have been developed and used, the demand for a circuit for performing easier and more stable startup and maintaining generated frequencies is rising.